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Old 09-08-2011, 06:12 PM   #214 (permalink)
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Originally Posted by ChazInMT View Post
Attached Flow Is Not The Goal.
You are correct, drag reduction while maintaining usability is the goal.
There are plenty of times that flow is attached while far short of the design template. Just because it is attached, it does not mean the shape is optimal.
Correct, air can stay attached up to about 30 degrees, which is great for keeping the rear windshield clean on lots of fastback cars, but terrible for their Cd!
So what is the goal? I say it is to let the air which has been pushed up by the front of the truck, thus building density (not pressure, but density), re-expand or return to normal density while pressing down on the skin on the back of the vehicle, thus returning its energy.
You will never have 100% of the air that went over the top fill in the wake, no matter what template you use. The air is at a higher density and velocity than the air in the wake and wants to move toward that area (or any lower velocity / lower density area) through the path of least resistance. If that happens to be up, down, left, or right, that's where it will move. Much will fill in the wake area, and that's what we are trying to maximize.
We optimize this reclaiming of energy when we follow the template.
Yes and no...
Yes, if you plan on taking your vehicle all the way out, following the template to completion, then yes, follow the template.
No, if you plan on cutting the vehicle anywhere short of the full template, use about a 12 degree angle. Hucho (and many others) have shown that for a truncated body, something like 10 or 12 degrees is the optimal for drag reduction. Granted, the tests done to generate these drag profile curves were done without making super smooth curves (like the template has), but they have been proven time and again to be about as good as you are going to get.
Look at these graphs, and ask yourself, where would the aerodynamic streamlining template have me be? Between 3.5 and 7.5 degrees? Following the logic above, this would be optimal, but it isn't... something more like 10 or 12 degrees wins the day.

Conversely, if we make the shape longer, then skin drag starts to become an issue.
Not to mention having your super long boat tail smash into things when turning!

I checked out your other post (reminding me that velocity was involved... tisk tisk on me!) and I will be commenting once I am done reading.

Last edited by wyatt; 09-08-2011 at 07:10 PM.. Reason: Bernoulli - Pressure, Density, and Velocity... shame on me!
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